Proton exchange membrane fuel cells have shown great promise as an energy source for devices ranging from hand held electronics, automobiles, to small scale fixed power units due to their large energy density, low temperature operation, and inert reaction products. A fuel cell (FC) operates on the principle of extracting energy from the conversion of high energy state reactant molecules to lower energy state product molecules via catalysts. For hydrogen fuel cells (HFC's), hydrogen is combined with oxygen to form water, heat, and electrical energy. The chemical reactions involved take place at two catalyst sites, the anode and cathode. A HFC produces electrical energy without producing greenhouse gases or pollution.
Proton exchange membrane fuel cells typically have an anode, an ion selective membrane, and a cathode. The anode and cathode usually have a hydrophobic gas diffusion layer, a catalyst layer, and a current collection layer. The ion selective membrane is designed to allow the transport of protons and has high resistance to electron conduction and transport of anions.
The net chemical reactions for HFC's are,2H2—4H++4e−  [1]4H++4e−+O2—2H2O  [2]
Eq. [1] occurs at the anode catalyst layer and Eq. [2] occurs at the cathode catalyst layer. While the basic principles of HFC operation are relatively straightforward, fuel cells have practical operational issues that limit their performance.
FIG. 1 shows an example of a prior art hydrogen fuel cell 100. The fuel cell 100 comprises a reactant duct 102, an anode 104, an ion permeable membrane 106, a cathode 108, and an oxidant duct 110. The membrane 106 is a poor electrical conductor and electrons travel through the external load 114 producing electrical power. An electrical current 112 travels from the anode 104 through the load 114 to the cathode 108. The load 114 is the device that the fuel cell is powering, such as a battery, an electric motor or an electronic device. The input reactant 116, in this case hydrogen, enters the reactant duct 102. The input reactant 116 can be pure dry hydrogen or hydrogen humidified with water vapor. The unused input reactant 116 exits as the output reactant 120. Ideally, in the case of a hydrogen fuel cell 100, the output reactant stream 120 would have small partial pressure of hydrogen as compared to the input reactant 116. The input oxidant 118 enters the oxidant duct 110. The oxidant stream 118 is pure oxygen, surrounding air with some fraction of oxygen, or one of the aforementioned streams humidified with water vapor. Unused oxidant and the unconsumed carrier gases (in the case of air: N2, CO2, Ar, etc.) exit the oxidant duct 110 along with product water 124 as the oxidant output stream 122. The product water may leave the fuel cell as both a vapor or a liquid depending on the thermodynamic conditions.
For the purposes of this application, oxygen enriched air (which includes pure oxygen) may be used interchangeably with regular air as an oxidant source. Those skilled in the art realize that the oxidant flow rate of pure oxygen will be roughly one fifth that of standard air to achieve the same fuel cell current density. Thus, when a figure shows O2, the oxidant supply may be either oxygen enriched air or regular air.
One example of a suitable membrane 106 is an ion exchange polymer or polymer exchange membrane (PEM) made from polyperfluorosulfonic acid (available as Nafion membrane by DuPont, USA). Ion transport occurs along pathways of ionic networks established by the anionic (sulfonic acid anion) groups that exist within the polymer. Liquid water is desired around ionic sites in the polymer to form conductive pathways for ionic transport. The ionic conductivity of this type of the PEM 106 is therefore dependent on proper hydration of the membrane. The ionic conductivity of the membrane 106 increases with water content. Optimum hydration of the membrane 106 is important to fuel cell performance. For this reason, water vapor is often carried in the reactant streams to prevent drying out of the PEM membrane.
Several transport mechanisms affect hydration of the membrane 106. The water transport mechanisms in typical fuel cells are evaporation, condensation, diffusion, and electroosmotic drag. The evaporation and condensation rates of water between the membrane 106 and the reactant streams (116, 118) depend on the (1) the partial pressures of water vapor in the reactant streams, (2) the gas and membrane temperatures, (3) the gas flow rates and velocities, and (4) the hydration state of the membrane. Typically, reactant streams are humidified to inhibit the PEM from drying out. During operation, water is electroosmotically dragged from the anode 104 through the PEM 106 towards the cathode 108 by hydrogen water compounds (for example, hydronium compounds such as (H3O)+). In this process, water molecules are dragged through the membrane 106 by hydrogen protons. Studies suggest that each hydrogen ion transport induces the transport of 1-5 water molecules towards the cathode 108. Molecular diffusion results in a flux of water aligned with negative concentration gradients within the membrane 106. Since water is electroosmotically dragged towards the cathode and water is produced at the cathode, molecular diffusion typically results in some diffusive transport of water back towards the anode. Maintaining a proper level of hydration of the membrane 106 at all times is challenging as water transport mechanisms are strongly coupled. Membrane hydration can vary spatially even within a single fuel cell flow structure. Some systems use long, serpentine-like oxidant channels to drive out water. In such devices, the fraction of water content along the channel length increases steadily in the direction of the outlet.
Another common type of fuel cell is the direct methanol fuel cell (DMFC). A DMFC uses a methanol-water mixture as a reactant stream. The cathode side of the DMFC works the same as for a HFC. The net chemical reactions in a DMFC are summarized in the following equations:CH3OH+H2O_CO2+6H++6e−  [3]6H++6e−+ 3/2O2—3H2O  [4]
Eq. [3] occurs at the anode catalyst layer and Eq. [4] occurs at the cathode catalyst layer. Advantages of DMFC's include: higher energy density than H2, ease of storage, and rapid refueling. These advantages stem from the fact that methanol is primarily a liquid at room temperature and pressure. Disadvantages of DMFC's include: CO2 product gases and reduced power density. DFMC's are primarily being developed for portable electronic devices.
FIG. 2 shows an example of a prior art direct methanol fuel cell 200. The fuel cell 200 is similar to the hydrogen fuel cell 100 shown in FIG. 1, except that the reactant input 216 comprises liquid methanol and water. The mixture of unconsumed reactants and carbon dioxide products 220 leaves the anode region via a duct 102. As in HFC's, DMFC also use air as the oxidant stream thus N2 and other trace gases will be present in the oxidant streams. The anode exit stream 220 will be methanol, depleted water, and CO2.
In both HFC's and DMFC's the product water at the cathode 108 can inhibit oxygen transport and reduce cell potential at higher current densities. One current method of dealing with product water is to remove the water with the oxidant stream. This method employs interdigitated flow distributors or serpentine channels to reduce the effect of electrode flooding. Experiments have shown that 2-60 times the stoichiometric rate of oxidant is typically used to reduce the detrimental affects of flooding. Serpentine and interdigitated channels generate large pressure drops and require large parasitic oxidant pumping powers. The oxidant pump power is drawn from the fuel cell and reduces the net power output of the fuel cell. Typically for kW sized fuel cells, 25% of the fuel cell power is lost to parasitic equipment such as oxidant pumps.